The function of a voltage regulator circuit is basically to maintain a precise voltage regardless of the current drawn by a load. FIG. 1 shows an example of a voltage regulator. This example of a voltage regulator comprises three basic components: a reference voltage source 3 (e.g. a band-gap reference) for generating a reference voltage Vref, an error amplifier circuit 4 which compares an output voltage Vout with the reference voltage Vref via the feedback resistors R1, R2 so as to determine the error in the output voltage, and an output stage 5 which regulates the output voltage according to the error amplifier output. A first resistor R1 is coupled to a node between the output stage 5 and the output port 2 of the voltage regulator and a second resistor R2 is serially coupled to the first resistor R1. An electric path is coupled to a node between the resistors R1 and R2 to tap-off a feedback voltage Vfb. The electric path and the two resistors R1 and R2 thus form a potential divider. The feedback voltage Vfb is always a particular fraction of the regulator output Vout. The error amplifier circuit further comprises an operational amplifier 4.1. A negative input of the operational amplifier 4.1 is supplied with the reference voltage Vref and a positive input of the operational amplifier 4.1 is supplied with the feedback voltage Vfb. The output of the operational amplifier 4.1 is coupled to a gate of the output stage 5.
Linear voltage regulators may require an input voltage at least some minimum amount higher than the desired output voltage. This minimum amount is called the “drop-out” voltage which is thus an important parameter of a voltage regulator. For example, a common voltage regulator has an output voltage of 5 V, but can only maintain it if the input voltage remains above about 7 V. Its drop-out voltage is therefore 7 V-5 V=2 V. When the supply voltage is less than about 2 V above the desired output voltage, the supply voltage to the regulator becomes too close to the output voltage such that the regulation properties start to degrade and the regulator can no longer hold the output voltage stable against changes in the input voltage.
There can be typically certain inaccuracies in the output voltage. An important contributing factor to this is the inaccuracy of the reference voltage Vref, particularly when using ultra low power band-gap references. Therefore, in a conventional linear regulator the minimum input voltage Vin is defined by the maximum output voltage Vout plus the drop-out voltage.
It is desirable to minimize the minimum allowable input voltage Vin. This allows a device where the regulator is connected to the battery to be operated for a longer portion of the battery discharge curve. In a system where a switched DC-DC converter supplies the input Vin, it allows the losses in the linear regulator to be minimized by minimizing the voltage drop across the regulator. This may imply that the drop-out voltage of the regulator must be kept as low as possible. However, a very low drop-out voltage may require an extremely low-resistance and physically large output device, which is undesirable in a low-cost circuit.